Biocompatible implants such as artificial bones and dental roots are attracting increasing attention of researchers because if bones are fractured or teeth come out as a result of accidents or for some other reason, such implants can be joined to the remaining bone or implanted in either the lower or upper mandible so as to restore the lost bone or tooth and enable the patient to continue a comfortable life. In addition to being nontoxic to humans, such implants which are intended to be embedded in the human body must satisfy many other stringent requirements such as high strength, good machinability, non-dissolvability, appropriate values of specific gravity, and biocompatibility.
Metals such as noble metals, metal alloys such as stainless steels and ceramics such as .alpha.-alumina have heretofore been used as implants but these materials have at least one of the disadvantages of being toxic, having poor strength, having no machinability, and undergoing dissolution. In addition, they have on common problem, viz., the lack of biocompatibility.
Apatite ceramics have recently been proposed as implants that have solved the biocompatibility problem. The inorganic components of bones and teeth are calcium phosphate compounds (which are chiefly made of hydroxyapatite) and the principal components of apatite ceramics are also calcium phosphate compounds. Therefore, apatite ceramics have a very good affinity for bones and teeth and guarantee very satisfactory integration in the human body after their implantation. However, the use of such apatite ceramics is presently very limited since they have defects similar to those of the previously developed materials, such as low strength and poor machinability.
In order to solve all of the problems described above, it is strongly desired to develop metal or ceramic composite materials that are provided with biocompatibility by applying apatite coatings to the surfaces of metals or ceramics. To achieve this end, metal-to-ceramics or ceramics-to-ceramics bonding techniques are necessary but plasma spraying is the only method known to date to accomplish this type of bonding. In spite of its utility for the purpose of bonding metals to ceramics or ceramics to ceramics, the plasma spray method has the following disadvantages: it is extremely difficult to form a uniform coating over the entire surface of a material having a complex shape; it is inherently incapable of forming a coating that covers the entire surface of a porous material; it requires the use of expensive equipment; it is not capable of efficient use of costly apatite particles in coatings; and it does not always produce a strong bond between the apatite coating and the substrate.